Development of the mammalian axial skeleton requires signaling through the Gα(i) subfamily of heterotrimeric G proteins

The Gq/11 family of Gα subunits is necessary and sufficient for lower jaw development

Vertebrate jaw development is coordinated by highly conserved ligand-receptor systems such as the peptide ligand Endothelin 1 (Edn1) and Endothelin receptor type A (Ednra), which are required for patterning of lower jaw structures. The Edn1/Ednra signaling pathway establishes the identity of lower jaw progenitor cells by regulating expression of numerous patterning genes, but the intracellular signaling mechanisms linking receptor activation to gene regulation remain poorly understood. As a first step towards elucidating this mechanism, we examined the function of the Gq/11 family of Gα subunits in zebrafish using pharmacological inhibition and genetic ablation of Gq/11 activity, and transgenic induction of a constitutively active Gq protein in edn1-/- embryos. Genetic loss of Gq/11 activity fully recapitulated the edn1-/- phenotype, with genes encoding G11 being most essential. Furthermore, inducing Gq activity in edn1-/- embryos not only restored Edn1/Ednra-dependent jaw structures and gene expression signatures but also caused homeosis of the upper jaw structure into a lower jaw-like structure. These results indicate that Gq/11 is necessary and sufficient to mediate the lower jaw patterning mechanism for Ednra in zebrafish.

The Gq/11 family of Gα subunits is necessary and sufficient for lower jaw development

Vertebrate jaw development is coordinated by highly conserved ligand-receptor systems such as the peptide ligand Endothelin 1 (Edn1) and Endothelin receptor type A (Ednra), which are required for patterning of lower jaw structures. The Edn1/Ednra signaling pathway establishes the identity of lower jaw progenitor cells by regulating expression of numerous patterning genes, but the intracellular signaling mechanisms linking receptor activation to gene regulation remain poorly understood. As a first step towards elucidating this mechanism, we examined the function of the Gq/11 family of Gα subunits in zebrafish using pharmacological inhibition and genetic ablation of Gq/11 activity and transgenic induction of a constitutively active Gq protein in edn1 -/- embryos. Genetic loss of Gq/11 activity fully recapitulated the edn1 -/- phenotype, with genes encoding G11 being most essential. Furthermore, inducing Gq activity in edn1 -/- embryos not only restored Edn1/Ednra-dependent jaw structures and gene expression signatures but also caused homeosis of the upper jaw structure into a lower jaw-like structure. These results indicate that Gq/11 is necessary and sufficient to mediate the lower jaw patterning mechanism for Ednra in zebrafish.

Inhibitory G proteins play multiple roles to polarize sensory hair cell morphogenesis

Inhibitory G alpha (GNAI or Gαi) proteins are critical for the polarized morphogenesis of sensory hair cells and for hearing. The extent and nature of their actual contributions remains unclear, however, as previous studies did not investigate all GNAI proteins and included non-physiological approaches. Pertussis toxin can downregulate functionally redundant GNAI1, GNAI2, GNAI3, and GNAO proteins, but may also induce unrelated defects. Here, we directly and systematically determine the role(s) of each individual GNAI protein in mouse auditory hair cells. GNAI2 and GNAI3 are similarly polarized at the hair cell apex with their binding partner G protein signaling modulator 2 (GPSM2), whereas GNAI1 and GNAO are not detected. In Gnai3 mutants, GNAI2 progressively fails to fully occupy the sub-cellular compartments where GNAI3 is missing. In contrast, GNAI3 can fully compensate for the loss of GNAI2 and is essential for hair bundle morphogenesis and auditory function. Simultaneous inactivation of Gnai2 and Gnai3 recapitulates for the first time two distinct types of defects only observed so far with pertussis toxin: (1) a delay or failure of the basal body to migrate off-center in prospective hair cells, and (2) a reversal in the orientation of some hair cell types. We conclude that GNAI proteins are critical for hair cells to break planar symmetry and to orient properly before GNAI2/3 regulate hair bundle morphogenesis with GPSM2.

Protective effects of Gαi3 deficiency in a murine heart-failure model of β1-adrenoceptor overexpression

We have shown that in murine cardiomyopathy caused by overexpression of the β1-adrenoceptor, Gαi2-deficiency is detrimental. Given the growing evidence for isoform-specific Gαi-functions, we now examined the consequences of Gαi3 deficiency in the same heart-failure model. Mice overexpressing cardiac β1-adrenoceptors with (β1-tg) or without Gαi3-expression (β1-tg/Gαi3-/-) were compared to C57BL/6 wildtypes and global Gαi3-knockouts (Gαi3-/-). The life span of β1-tg mice was significantly shortened but improved when Gαi3 was lacking (95% CI: 592-655 vs. 644-747 days). At 300 days of age, left-ventricular function and survival rate were similar in all groups. At 550 days of age, β1-tg but not β1-tg/Gαi3-/- mice displayed impaired ejection fraction (35 ± 18% vs. 52 ± 16%) compared to wildtype (59 ± 4%) and Gαi3-/- mice (60 ± 5%). Diastolic dysfunction of β1-tg mice was prevented by Gαi3 deficiency, too. The increase of ANP mRNA levels and ventricular fibrosis observed in β1-tg hearts was significantly attenuated in β1-tg/Gαi3-/- mice. Transcript levels of phospholamban, ryanodine receptor 2, and cardiac troponin I were similar in all groups. However, Western blots and phospho-proteomic analyses showed that in β1-tg, but not β1-tg/Gαi3-/- ventricles, phospholamban protein was reduced while its phosphorylation increased. Here, we show that in mice overexpressing the cardiac β1-adrenoceptor, Gαi3 deficiency slows or even prevents cardiomyopathy and increases shortened life span. Previously, we found Gαi2 deficiency to aggravate cardiac dysfunction and mortality in the same heart-failure model. Our findings indicate isoform-specific interventions into Gi-dependent signaling to be promising cardio-protective strategies.

Non-canonical G protein signaling

The original paradigm of classical - also referred to as canonical - cellular signal transduction of heterotrimeric G proteins (G protein) is defined by a hierarchical, orthograde interaction of three players: the agonist-activated G protein-coupled receptor (GPCR), which activates the transducing G protein, that in turn regulates its intracellular effectors. This receptor-transducer-effector concept was extended by the identification of regulators and adapters such as the regulators of G protein signaling (RGS), receptor kinases like βARK, or GPCR-interacting arrestin adapters that are integrated into this canonical signaling process at different levels to enable fine-tuning. Finally, the identification of atypical signaling mechanisms of classical regulators, together with the discovery of novel modulators, added a new and fascinating dimension to the cellular G protein signal transduction. This heterogeneous group of accessory G protein modulators was coined "activators of G protein signaling" (AGS) proteins and plays distinct roles in canonical and non-canonical G protein signaling pathways. AGS proteins contribute to the control of essential cellular functions such as cell development and division, intracellular transport processes, secretion, autophagy or cell movements. As such, they are involved in numerous biological processes that are crucial for diseases, like diabetes mellitus, cancer, and stroke, which represent major health burdens. Although the identification of a large number of non-canonical G protein signaling pathways has broadened the spectrum of this cellular communication system, their underlying mechanisms, functions, and biological effects are poorly understood. In this review, we highlight and discuss atypical G protein-dependent signaling mechanisms with a focus on inhibitory G proteins (Gi) involved in canonical and non-canonical signal transduction, review recent developments and open questions, address the potential of new approaches for targeted pharmacological interventions.

Inhibitory G proteins play multiple roles to polarize sensory hair cell morphogenesis

Inhibitory G alpha (GNAI or Gαi) proteins are critical for the polarized morphogenesis of sensory hair cells and for hearing. The extent and nature of their actual contributions remains unclear, however, as previous studies did not investigate all GNAI proteins and included non-physiological approaches. Pertussis toxin can downregulate functionally redundant GNAI1, GNAI2, GNAI3 and GNAO proteins, but may also induce unrelated defects. Here we directly and systematically determine the role(s) of each individual GNAI protein in mouse auditory hair cells. GNAI2 and GNAI3 are similarly polarized at the hair cell apex with their binding partner GPSM2, whereas GNAI1 and GNAO are not detected. In Gnai3 mutants, GNAI2 progressively fails to fully occupy the subcellular compartments where GNAI3 is missing. In contrast, GNAI3 can fully compensate for the loss of GNAI2 and is essential for hair bundle morphogenesis and auditory function. Simultaneous inactivation of Gnai2 and Gnai3 recapitulates for the first time two distinct types of defects only observed so far with pertussis toxin: 1) a delay or failure of the basal body to migrate off-center in prospective hair cells, and 2) a reversal in the orientation of some hair cell types. We conclude that GNAI proteins are critical for hair cells to break planar symmetry and to orient properly before GNAI2/3 regulate hair bundle morphogenesis with GPSM2.

Genome-Wide Association Studies for Body Conformation Traits in Korean Holstein Population

The objective of this study was to identify quantitative trait loci (QTL) and nearby candidate genes that influence body conformation traits. Phenotypic data for 24 body conformation traits were collected from a population of 2329 Korean Holstein cattle, and all animals were genotyped using the 50 K Illumina bovine SNP chip. A total of 24 genome-wide significant SNPs associated with 24 body conformation traits were identified by genome-wide association analysis. The selection of the most promising candidate genes was based on gene ontology (GO) terms and the previously identified functions that influence various body conformation traits as determined in our study. These genes include KCNA1, RYBP, PTH1R, TMIE, and GNAI3 for body traits; ANGPT1 for rump traits; MALRD1, INHBA, and HOXA13 for feet and leg traits; and CDK1, RHOBTB1, and SLC17A1 for udder traits, respectively. These findings contribute to our understanding of the genetic basis of body conformation traits in this population and pave the way for future breeding strategies aimed at enhancing desirable traits in dairy cattle.

Gαi protein subunit: A step toward understanding its non-canonical mechanisms

The heterotrimeric G protein family plays essential roles during a varied array of cellular events; thus, its deregulation can seriously alter signaling events and the overall state of the cell. Heterotrimeric G-proteins have three subunits (α, β, γ) and are subdivided into four families, Gαi, Gα12/13, Gαq, and Gαs. These proteins cycle between an inactive Gα-GDP state and active Gα-GTP state, triggered canonically by the G-protein coupled receptor (GPCR) and by other accessory proteins receptors independent also known as AGS (Activators of G-protein Signaling). In this review, we summarize research data specific for the Gαi family. This family has the largest number of individual members, including Gαi1, Gαi2, Gαi3, Gαo, Gαt, Gαg, and Gαz, and constitutes the majority of G proteins α subunits expressed in a tissue or cell. Gαi was initially described by its inhibitory function on adenylyl cyclase activity, decreasing cAMP levels. Interestingly, today Gi family G-protein have been reported to be importantly involved in the immune system function. Here, we discuss the impact of Gαi on non-canonical effector proteins, such as c-Src, ERK1/2, phospholipase-C (PLC), and proteins from the Rho GTPase family members, all of them essential signaling pathways regulating a wide range of physiological processes.

Further delineation of Auriculocondylar syndrome based on 14 novel cases and reassessment of 25 published cases

Auriculocondylar syndrome (ACS) is a rare craniofacial disorder characterized by mandibular hypoplasia and an auricular defect at the junction between the lobe and helix, known as a "Question Mark Ear" (QME). Several additional features, originating from the first and second branchial arches and other tissues, have also been reported. ACS is genetically heterogeneous with autosomal dominant and recessive modes of inheritance. The mutations identified to date are presumed to dysregulate the endothelin 1 signalling pathway. Here we describe 14 novel cases and reassess 25 published cases of ACS through a questionnaire for systematic data collection. All patients harbour mutation(s) in PLCB4, GNAI3 or EDN1. This series of patients contributes to the characterization of additional features occasionally associated with ACS such as respiratory, costal, neurodevelopmental and genital anomalies, and provides management and monitoring recommendations. This article is protected by copyright. All rights reserved.

Guanine Nucleotide-Binding Protein G(i) Subunit Alpha 2 Exacerbates NASH Progression by Regulating Peroxiredoxin 1-Related Inflammation and Lipophagy

BACKGROUND AND AIMS

NASH is an advanced stage of liver disease accompanied by lipid accumulation, inflammation, and liver fibrosis. Guanine nucleotide-binding protein G(i) subunit alpha-2 (GNAI2) is a member of the "inhibitory" class of α-subunits, and recent studies showed that Gnai2 deficiency is known to cause reduced weight in mice. However, the role of GNAI2 in hepatocytes, particularly in the context of liver inflammation and lipid metabolism, remains to be elucidated. Herein, we aim to ascertain the function of GNAI2 in hepatocytes and its impact on the development of NASH.

APPROACH AND RESULTS

Human liver tissues were obtained from NASH patients and healthy persons to evaluate the expression and clinical relevance of GNAI2. In addition, hepatocyte-specific Gnai2-deficient mice (Gnai2^hep-/- ) were fed either a Western diet supplemented with fructose in drinking water (WDF) for 16 weeks or a methionine/choline-deficient diet (MCD) for 6 weeks to investigate the regulatory role and underlying mechanism of Gnai2 in NASH. GNAI2 was significantly up-regulated in liver tissues of patients with NASH. Following feeding with WDF or MCD diets, livers from Gnai2^hep-/- mice had reduced steatohepatitis with suppression of markers of inflammation and an increase in lipophagy compared to Gnai2^flox/flox mice. Toll-like receptor 4 signals through nuclear factor kappa B to trigger p65-dependent transcription of Gnai2. Intriguingly, immunoprecipitation, immunofluorescence, and mass spectrometry identified peroxiredoxin 1 (PRDX1) as a binding partner of GNAI2. Moreover, the function of PRDX1 in the suppression of TNF receptor-associated factor 6 ubiquitin-ligase activity and glycerophosphodiester phosphodiesterase domain-containing 5-related phosphatidylcholine metabolism was inhibited by GNAI2. Suppression of GNAI2 combined with overexpression of PRDX1 reversed the development of steatosis and fibrosis in vivo.

CONCLUSIONS

GNAI2 is a major regulator that leads to the development of NASH. Thus, inhibition of GNAI2 could be an effective therapeutic target for the treatment of NASH.

Analyses of Gnai3-iresGFP reporter mice reveal unknown Gαi3 expression sites

Inhibitory G proteins (Gi proteins) are highly homologous but play distinct biological roles. However, their isoform-specific detection remains challenging. To facilitate the analysis of Gαi3 expression, we generated a Gnai3- iresGFP reporter mouse line. An internal ribosomal entry site (IRES) was inserted behind the stop-codon of the Gnai3 gene to initiate simultaneous translation of the GFP cDNA together with Gαi3. The expression of GFP was confirmed in spleen and thymus tissue by immunoblot analysis. Importantly, the GFP knock-in (ki) did not alter Gαi3 expression levels in all organs tested including spleen and thymus compared to wild-type littermates. Flow cytometry of thymocytes, splenic and blood cell suspensions revealed significantly higher GFP fluorescence intensities in homozygous ki/ki animals compared to heterozygous mice (+/ki). Using cell-type specific surface markers GFP fluorescence was assigned to B cells, T cells, macrophages and granulocytes from both splenic and blood cells and additionally blood-derived platelets. Moreover, immunofluorescent staining of the inner ear from knock-in mice unraveled GFP expression in sensory and non-sensory cell types, with highest levels in Deiter's cells and in the first row of Hensen's cells in the organ of Corti, indicating a novel site for Gαi3 expression. In summary, the Gnai3- iresGFP reporter mouse represents an ideal tool for precise analyses of Gαi3 expression patterns and sites.

Functional approaches to the study of G-protein-coupled receptors in postmortem brain tissue: [35S]GTPγS binding assays combined with immunoprecipitation

G-protein-coupled receptors (GPCRs) have an enormous biochemical importance as they bind to diverse extracellular ligands and regulate a variety of physiological and pathological responses. G-protein activation measures the functional consequence of receptor occupancy at one of the earliest receptor-mediated events. Receptor coupling to G-proteins promotes the GDP/GTP exchange on Gα subunits. Thus, modulation of the binding of the poorly hydrolysable GTP analog [³⁵S]GTPγS to the Gα-protein subunit can be used as a functional approach to quantify GPCR interaction with agonist, antagonist or inverse agonist drugs. In order to determine receptor-mediated selective activation of the different Gα-proteins, [³⁵S]GTPγS binding assays combined with immunodetection by specific antibodies have been developed and applied to physiological and pathological brain conditions. Currently, immunoprecipitation with magnetic beads and scintillation proximity assays are the most habitual techniques for this purpose. The present review summarizes the different procedures, advantages and limitations of the [³⁵S]GTPγS binding assays combined with selective Gα-protein sequestration methods. Experience of functional coupling of several GPCRs to different Gα-proteins and recommendations for optimal performance in brain membranes are described. One of the biggest opportunities opened by these techniques is that they enable evaluation of biased agonism in the native tissue, which results in high interest in drug discovery. The available results derived from application of these functional methodologies to study GPCR dysfunctions in neuro-psychiatric disorders are also described. In conclusion, [³⁵S]GTPγS binding combined with antibody-mediated immunodetection represents an useful method to separately evaluate the functional activity of drugs acting on GPCRs over each Gα-protein subtype.

Lack of Gαi2 proteins in adipocytes attenuates diet-induced obesity

OBJECTIVES

Typically, obesity results from an inappropriate balance between energy uptake from nutrient consumption and burning of calories, which leads to a pathological increase in fat mass. Obesity is a major cause of insulin resistance and diabetes. Inhibitory G proteins (Gαi) form a subfamily that is involved in the regulation of adipose tissue function. Among the three Gαi members, i.e. Gαi1, Gαi2, Gαi3, the Gαi2, protein is predominantly expressed in adipose tissue. However, the functions of the Gαi2 isoform in adipose tissue and its impact on the development of obesity are poorly understood.

METHODS

By using AdipoqCreERT2 mice, we generated adipocyte-specific Gnai2-deficient mice to study Gαi2 function, specifically in white and brown adipocytes. These mice were fed either a control diet (CD) or a high fat diet (HFD). Mice were examined for obesity development, insulin resistance and glucose intolerance. We examined adipocyte morphology and the development of inflammation in the white adipose tissue. Finally, intracellular cAMP levels as an indicator of Gαi signaling and glycerol release as an indicator of lipolysis rates were measured to verify the impact of Gαi2 on the signaling pathway in brown and white adipocytes.

RESULTS

An adipocyte-specific deficiency of Gαi2 significantly reduced diet-induced obesity, leading to decreased fat masses, smaller adipocytes and decreased inflammation in the white adipose tissue relative to littermate controls. Concurrently, oxygen consumption of brown adipocytes and in vivo measured energy expenditure were significantly enhanced. In addition, glucose tolerance and insulin sensitivity of HFD-fed adipocyte-specific Gnai2-deficient mice were improved compared to the respective controls. In the absence of Gαi2, adrenergic stimulation of intracellular adipocyte cAMP levels was increased, which correlated with increased lipolysis and energy expenditure.

CONCLUSION

We conclude that adipocyte Gαi2 is a major regulator of adipocyte lipid content in diet-induced obesity by inhibiting adipocyte lipolysis in a cAMP-dependent manner resulting in increased energy expenditure.

Selective protection of murine cerebral Gi/o-proteins from inactivation by parenterally injected pertussis toxin

Pertussis toxin (PTX) is a potent virulence factor in patients suffering from whooping cough, but in its detoxified version, it is applied for vaccination. It is thought to contribute to the pathology of the disease including various CNS malfunctions. Based on its enzymatic activity, PTX disrupts GPCR-dependent signaling by modifying the α-subunit of heterotrimeric G_i/o-proteins. It is also extensively used as a research tool to study neuronal functions in vivo and in vitro. However, data demonstrating the penetration of PTX from the blood into the brain are missing. Here, we examined the Gα_i/o-modifying activity of PTX in murine brains after its parenteral application. Ex vivo biodistribution analysis of [¹²⁴I]-PTX displayed poor distribution to the brain while relatively high concentrations were visible in the pancreas. PTX affected CNS and endocrine functions of the pancreas as shown by open-field and glucose tolerance tests, respectively. However, while pancreatic islet Gα_i/o-proteins were modified, their neuronal counterparts in brain tissue were resistant towards PTX as indicated by different autoradiographic and immunoblot SDS-PAGE analyses. In contrast, PTX easily modified brain Gα_i/o-proteins ex vivo. An attempt to increase BBB permeability by application of hypertonic mannitol did not show PTX activity on neuronal G proteins. Consistent with these findings, in vivo MRI analysis did not point to an increased blood-brain barrier (BBB) permeability following PTX treatment. Our data demonstrate that the CNS is protected from PTX. Thus, we hypothesize that the BBB hinders PTX to penetrate into the CNS and to deliver its enzymatic activity to brain Gα_i/o-proteins. KEY MESSAGES: i.p. applied PTX is poorly retained in the brain while reaches high concentration in the pancreas. Pancreatic islet Gα_i/o- but not cerebral Gα_i/o-proteins are modified by i.p. administered PTX. Gα_i/o-proteins from isolated cerebral cell membranes were easily modified by PTX ex vivo. CNS is protected from i.p. administered PTX. PTX does not permeabilize the BBB.

Guanine nucleotide-binding protein G(i)α2 aggravates hepatic ischemia-reperfusion injury in mice by regulating MLK3 signaling

Hepatic ischemia-reperfusion (I/R) injury is a major challenge in liver resection and transplantation surgeries. Previous studies have revealed that guanine nucleotide-binding protein G(i)α2 (GNAI2) was involved in the progression of myocardial and cerebral I/R injury, but the role and function of GNAI2 in hepatic I/R have not been elucidated. The hepatocyte-specific GNAI2 knockout (GNAI2hep-/-) mice were generated and subjected to hepatic I/R injury. Primary hepatocytes isolated from GNAI2hep-/- and GNAI2flox/flox mice were cultured and challenged to hypoxia-reoxygenation insult. The specific function of GNAI2 in I/R-triggered hepatic injury and the underlying molecular mechanism were explored by various phenotypic analyses and molecular biology methods. In this study, we demonstrated that hepatic GNAI2 expression was significantly increased in liver transplantation patients and wild-type mice after hepatic I/R. Interestingly, hepatocyte-specific GNAI2 deficiency attenuated I/R-induced liver damage, inflammation cytokine expression, macrophage/neutrophil infiltration, and hepatocyte apoptosis in vivo and in vitro. Mechanistically, up-regulation of GNAI2 phosphorylates mixed-lineage protein kinase 3 (MLK3) through direct binding, which exacerbated hepatic I/R damage via MAPK and NF-κB pathway activation. Furthermore, blocking MLK3 signaling reversed GNAI2-mediated hepatic I/R injury. Our study firstly identifies GNAI2 as a promising target for prevention of hepatic I/R-induced injury and related liver diseases.-Sun, Q., He, Q., Xu, J., Liu, Q., Lu, Y., Zhang, Z., Xu, X., Sun, B. Guanine nucleotide-binding protein G(i)α2 aggravates hepatic ischemia-reperfusion injury in mice by regulating MLK3 signaling.

GNAI1 and GNAI3 Reduce Colitis-Associated Tumorigenesis in Mice by Blocking IL6 Signaling and Down-regulating Expression of GNAI2

BACKGROUND & AIMS

Interleukin 6 (IL6) and tumor necrosis factor contribute to the development of colitis-associated cancer (CAC). We investigated these signaling pathways and the involvement of G protein subunit alpha i1 (GNAI1), GNAI2, and GNAI3 in the development of CAC in mice and humans.

METHODS

B6;129 wild-type (control) or mice with disruption of Gnai1, Gnai2, and/or Gnai3 or conditional disruption of Gnai2 in CD11c+ or epithelial cells were given dextran sulfate sodium (DSS) to induce colitis followed by azoxymethane (AOM) to induce carcinogenesis; some mice were given an antibody against IL6. Feces were collected from mice, and the compositions of microbiomes were analyzed by polymerase chain reactions. Dendritic cells (DCs) and myeloid-derived suppressor cells (MDSCs) isolated from spleen and colon tissues were analyzed by flow cytometry. We performed immunoprecipitation and immunoblot analyses of colon tumor tissues, MDSCs, and mouse embryonic fibroblasts to study the expression levels of GNAI1, GNAI2, and GNAI3 and the interactions of GNAI1 and GNAI3 with proteins in the IL6 signaling pathway. We analyzed the expression of Gnai2 messenger RNA by CD11c+ cells in the colonic lamina propria by PrimeFlow, expression of IL6 in DCs by flow cytometry, and secretion of cytokines in sera and colon tissues by enzyme-linked immunosorbent assay. We obtained colon tumor and matched nontumor tissues from 83 patients with colorectal cancer having surgery in China and 35 patients with CAC in the United States. Mouse and human colon tissues were analyzed by histology, immunoblot, immunohistochemistry, and/or RNA-sequencing analyses.

RESULTS

GNAI1 and GNAI3 (GNAI1;3) double-knockout (DKO) mice developed more severe colitis after administration of DSS and significantly more colonic tumors than control mice after administration of AOM plus DSS. Development of increased tumors in DKO mice was not associated with changes in fecal microbiomes but was associated with activation of nuclear factor (NF) κB and signal transducer and activator of transcription (STAT) 3; increased levels of GNAI2, nitric oxide synthase 2, and IL6; increased numbers of CD4+ DCs and MDSCs; and decreased numbers of CD8+ DCs. IL6 was mainly produced by CD4+/CD11b+, but not CD8+, DCs in DKO mice. Injection of DKO mice with a blocking antibody against IL6 reduced the expansion of MDSCs and the number of tumors that developed after CAC induction. Incubation of MDSCs or mouse embryonic fibroblasts with IL6 induced activation of either NF-κB by a JAK2-TRAF6-TAK1-CHUK/IKKB signaling pathway or STAT3 by JAK2. This activation resulted in expression of GNAI2, IL6 signal transducer (IL6ST, also called GP130) and nitric oxide synthase 2, and expansion of MDSCs; the expression levels of these proteins and expansion of MDSCs were further increased by the absence of GNAI1;3 in cells and mice. Conditional disruption of Gnai2 in CD11c+ cells of DKO mice prevented activation of NF-κB and STAT3 and changes in numbers of DCs and MDSCs. Colon tumor tissues from patients with CAC had reduced levels of GNAI1 and GNAI3 and increased levels of GNAI2 compared with normal tissues. Further analysis of a public human colorectal tumor DNA microarray database (GSE39582) showed that low Gani1 and Gnai3 messenger RNA expression and high Gnai2 messenger RNA expression were significantly associated with decreased relapse-free survival.

CONCLUSIONS

GNAI1;3 suppresses DSS-plus-AOM-induced colon tumor development in mice, whereas expression of GNAI2 in CD11c+ cells and IL6 in CD4+/CD11b+ DCs appears to promote these effects. Strategies to induce GNAI1;3, or block GNAI2 and IL6, might be developed for the prevention or therapy of CAC in patients.

GPCR-independent activation of G proteins promotes apical cell constriction in vivo

Heterotrimeric G proteins are signaling switches that control organismal morphogenesis across metazoans. In invertebrates, specific GPCRs instruct G proteins to promote collective apical cell constriction in the context of epithelial tissue morphogenesis. In contrast, tissue-specific factors that instruct G proteins during analogous processes in vertebrates are largely unknown. Here, we show that DAPLE, a non-GPCR protein linked to human neurodevelopmental disorders, is expressed specifically in the neural plate of Xenopus laevis embryos to trigger a G protein signaling pathway that promotes apical cell constriction during neurulation. DAPLE localizes to apical cell-cell junctions in the neuroepithelium, where it activates G protein signaling to drive actomyosin-dependent apical constriction and subsequent bending of the neural plate. This function is mediated by a Gα-binding-and-activating (GBA) motif that was acquired by DAPLE in vertebrates during evolution. These findings reveal that regulation of tissue remodeling during vertebrate development can be driven by an unconventional mechanism of heterotrimeric G protein activation that operates in lieu of GPCRs.

Gαi2 Signaling Regulates Inflammasome Priming and Cytokine Production by Biasing Macrophage Phenotype Determination

Macrophages exist as innate immune subsets that exhibit phenotypic heterogeneity and functional plasticity. Their phenotypes are dictated by inputs from the tissue microenvironment. G-protein-coupled receptors are essential in transducing signals from the microenvironment, and heterotrimeric Gα signaling links these receptors to downstream effectors. Several Gα_i-coupled G-protein-coupled receptors have been implicated in macrophage polarization. In this study, we use genetically modified mice to investigate the role of Gα_i2 on inflammasome activity and macrophage polarization. We report that Gα_i2 in murine bone marrow-derived macrophages (BMDMs) regulates IL-1β release after activation of the NLRP3, AIM2, and NLRC4 inflammasomes. We show this regulation stems from the biased polarity of Gα_i2 deficient (Gnai2 ^-/-) and RGS-insensitive Gα_i2 (Gnai2 ^G184S/G184S) BMDMs. We determined that although Gnai2 ^G184S/G184S BMDMs (excess Gα_i2 signaling) have a tendency toward classically activated proinflammatory (M1) phenotype, Gnai2^-/- BMDMs (Gα_i2 deficient) are biased toward alternatively activated anti-inflammatory (M2) phenotype. Finally, we find that Gα_i2-deficient macrophages have increased Akt activation and IFN-β production but defects in ERK1/2 and STAT3 activation after LPS stimulation. Gα_i2-deficient macrophages also exhibit increased STAT6 activation after IL-4 stimulation. In summary, our data indicates that excess Gα_i2 signaling promotes an M1 macrophage phenotype, whereas Gα_i2 signaling deficiency promotes an M2 phenotype. Understanding Gα_i2-mediated effects on macrophage polarization may bring to light insights regarding disease pathogenesis and the reprogramming of macrophages for the development of novel therapeutics.

Gαi3 signaling is associated with sexual dimorphic expression of the clock-controlled output gene Dbp in murine liver

The albumin D-box binding protein (DBP) is a member of the PAR bZip (proline and acidic amino acid-rich basic leucine zipper) transcription factor family and functions as important regulator of circadian core and output gene expression. Gene expression of DBP itself is under the control of E-box-dependent binding by the Bmal1-Clock heterodimer and CRE-dependent binding by the cAMP responsive element binding protein (CREB). However, the signaling mechanism mediating CREB-dependent regulation of DBP expression in the peripheral clock remains elusive. In this study, we examined the role of the GPCR (G-protein-coupled receptor)/Gα_i3 (Galphai3) controlled cAMP-CREB signaling pathway in the regulation of hepatic expression of core clock and clock-regulated genes, including Dbp. Analysis of circadian gene expression revealed that rhythmicity of hepatic transcript levels of the majority of core clock (including Per1) and clock-regulated genes were not affected by Gα_i3 deficiency. Consistently, the period length of primary Gα_i3 deficient tail fibroblasts expressing a Bmal1-Luciferase reporter was not affected. Interestingly, however, Gα_i3 deficient female but not male mice showed a tendentiously increased activation of CREB (nuclear pSer133-CREB) accompanied by an advanced peak in Dbp gene expression and elevated mRNA levels of the cytochrome P₄₅₀ family member Cyp3a11, a target gene of DBP. Accordingly, selective inhibition of CREB led to a strongly decreased expression of DBP and CYP3A4 (human Cyp3a11 homologue) in HepG2 liver cells. In summary, our data suggest that the Gα_i3-pCREB signalling pathway functions as a regulator of sexual-dimorphic expression of DBP and its xenobiotic target enzymes Cyp3a11/CYP3A4.

Genetic Analysis of Rare Human Variants of Regulators of G Protein Signaling Proteins and Their Role in Human Physiology and Disease

Regulators of G protein signaling (RGS) proteins modulate the physiologic actions of many neurotransmitters, hormones, and other signaling molecules. Human RGS proteins comprise a family of 20 canonical proteins that bind directly to G protein-coupled receptors/G protein complexes to limit the lifetime of their signaling events, which regulate all aspects of cell and organ physiology. Genetic variations account for diverse human traits and individual predispositions to disease. RGS proteins contribute to many complex polygenic human traits and pathologies such as hypertension, atherosclerosis, schizophrenia, depression, addiction, cancers, and many others. Recent analysis indicates that most human diseases are due to extremely rare genetic variants. In this study, we summarize physiologic roles for RGS proteins and links to human diseases/traits and report rare variants found within each human RGS protein exome sequence derived from global population studies. Each RGS sequence is analyzed using recently described bioinformatics and proteomic tools for measures of missense tolerance ratio paired with combined annotation-dependent depletion scores, and protein post-translational modification (PTM) alignment cluster analysis. We highlight selected variants within the well-studied RGS domain that likely disrupt RGS protein functions and provide comprehensive variant and PTM data for each RGS protein for future study. We propose that rare variants in functionally sensitive regions of RGS proteins confer profound change-of-function phenotypes that may contribute, in newly appreciated ways, to complex human diseases and/or traits. This information provides investigators with a valuable database to explore variation in RGS protein function, and for targeting RGS proteins as future therapeutic targets.

Gαi Proteins are Indispensable for Hearing

BACKGROUND/AIMS

From invertebrates to mammals, Gαi proteins act together with their common binding partner Gpsm2 to govern cell polarization and planar organization in virtually any polarized cell. Recently, we demonstrated that Gαi3-deficiency in pre-hearing murine cochleae pointed to a role of Gαi3 for asymmetric migration of the kinocilium as well as the orientation and shape of the stereociliary ("hair") bundle, a requirement for the progression of mature hearing. We found that the lack of Gαi3 impairs stereociliary elongation and hair bundle shape in high-frequency cochlear regions, linked to elevated hearing thresholds for high-frequency sound. How these morphological defects translate into hearing phenotypes is not clear.

METHODS

Here, we studied global and conditional Gnai3 and Gnai2 mouse mutants deficient for either one or both Gαi proteins. Comparative analyses of global versus Foxg1-driven conditional mutants that mainly delete in the inner ear and telencephalon in combination with functional tests were applied to dissect essential and redundant functions of different Gαi isoforms and to assign specific defects to outer or inner hair cells, the auditory nerve, satellite cells or central auditory neurons.

RESULTS

Here we report that lack of Gαi3 but not of the ubiquitously expressed Gαi2 elevates hearing threshold, accompanied by impaired hair bundle elongation and shape in high-frequency cochlear regions. During the crucial reprogramming of the immature inner hair cell (IHC) synapse into a functional sensory synapse of the mature IHC deficiency for Gαi2 or Gαi3 had no impact. In contrast, double-deficiency for Gαi2 and Gαi3 isoforms results in abnormalities along the entire tonotopic axis including profound deafness associated with stereocilia defects. In these mice, postnatal IHC synapse maturation is also impaired. In addition, the analysis of conditional versus global Gαi3-deficient mice revealed that the amplitude of ABR wave IV was disproportionally elevated in comparison to ABR wave I indicating that Gαi3 is selectively involved in generation of neural gain during auditory processing.

CONCLUSION

We propose a so far unrecognized complexity of isoform-specific and overlapping Gαi protein functions particular during final differentiation processes.

Loss of Gαi proteins impairs thymocyte development, disrupts T-cell trafficking, and leads to an expanded population of splenic CD4+PD-1+CXCR5+/- T-cells

Thymocyte and T cell trafficking relies on signals initiated by G-protein coupled receptors. To address the importance of the G-proteins Gα_i2 and Gα_i3 in thymocyte and T cell function, we developed several mouse models. Gα_i2 deficiency in hematopoietic progenitors led to a small thymus, a double negative (DN)1/DN2 thymocyte transition block, and an accumulation of mature single positive (SP) thymocytes. Loss at the double positive (DP) stage of thymocyte development caused an increase in mature cells within the thymus. In both models an abnormal distribution of memory and naïve CD4 T cells occurred, and peripheral CD4 and CD8 T cells had reduced chemoattractant responses. The loss of Gα_i3 had no discernable impact, however the lack of both G-proteins commencing at the DP stage caused a severe T cell phenotype. These mice lacked a thymic medullary region, exhibited thymocyte retention, had a peripheral T cell deficiency, and lacked T cell chemoattractant responses. Yet a noteworthy population of CD4⁺PD-1⁺CXCR5^+/− cells resided in the spleen of these mice likely due to a loss of regulatory T cell function. Our results delineate a role for Gα_i2 in early thymocyte development and for Gα_i2/3 in multiple aspects of T cell biology.

Role of GATA binding protein 4 (GATA4) in the regulation of tooth development via GNAI3

Transcription factor GATA4 regulates cardiac and osteoblast differentiation. However, its role in tooth development is not clear. Therefore, we generated Wnt1-Cre;GATA4^fl/fl mice, with conditional inactivation of the GATA4 gene in the dental papilla mesenchymal cells. Phenotypic analysis showed short root deformity along with reduced expressions of odonto/osteogenic markers. Proliferation (but not apoptosis) of cells around the apical area of the root was attenuated. In vitro, we knocked down GATA4 expression in stem cells of dental apical papilla (SCAPs). Proliferation, migration and odonto/osteogenic differentiation of SCAPs were affected in the shGATA4 group. Overexpression of GATA4 in SCAPs increased mineralization. Based on our previous iTRAQ results, guanine nucleotide binding proteins 3 (GNAI3) is one of the distinct proteins after GATA4 deletion. G protein signaling is involved in bone development, remodeling, and disease. In this study, both GATA4 deletion in the mouse root and knock-down in human SCAPs decreased the expression of GNAI3. Dual-luciferase and ChIP assay confirmed the direct binding of GATA4 to the GNAI3 promoter, both in vitro and in vivo. GNAI3 knock-down significantly decreased the odonto/osteogenic differentiation ability of SCAPs. We thus establish the role of GATA4 as a novel regulator of root development and elucidate its downstream molecular events.

Biased Agonism of Three Different Cannabinoid Receptor Agonists in Mouse Brain Cortex

Cannabinoid receptors are able to couple to different families of G proteins when activated by an agonist drug. It has been suggested that different intracellular responses may be activated depending on the ligand. The goal of the present study was to characterize the pattern of G protein subunit stimulation triggered by three different cannabinoid ligands, Δ⁹-THC, WIN55212-2, and ACEA in mouse brain cortex. Stimulation of the [³⁵S]GTPγS binding coupled to specific immunoprecipitation with antibodies against different subtypes of G proteins (Gα_i1, Gα_i2, Gα_i3, Gα_o, Gα_z, Gα_s, Gα_q/11, and Gα_12/13), in the presence of Δ⁹-THC, WIN55212-2 and ACEA (submaximal concentration 10 μM) was determined by scintillation proximity assay (SPA) technique in mouse cortex of wild type, CB₁ knock-out, CB₂ knock-out and CB₁/CB₂ double knock-out mice. Results show that, in mouse brain cortex, cannabinoid agonists are able to significantly stimulate not only the classical inhibitory Gα_i/o subunits but also other G subunits like Gα_z, Gα_q/11, and Gα_12/13. Moreover, the specific pattern of G protein subunit activation is different depending on the ligand. In conclusion, our results demonstrate that, in mice brain native tissue, different exogenous cannabinoid ligands are able to selectively activate different inhibitory and non-inhibitory Gα protein subtypes, through the activation of CB₁ and/or CB₂ receptors. Results of the present study may help to understand the specific molecular pathways involved in the pharmacological effects of cannabinoid-derived drugs.

An essential role for Gα(i2) in Smoothened-stimulated epithelial cell proliferation in the mammary gland

Hedgehog (Hh) signaling is critical for organogenesis, tissue homeostasis, and stem cell maintenance. The gene encoding Smoothened (SMO), the primary effector of Hh signaling, is expressed aberrantly in human breast cancer, as well as in other cancers. In mice that express a constitutively active form of SMO that does not require Hh stimulation in mammary glands, the cells near the transgenic cells proliferate and participate in hyperplasia formation. Although SMO is a seven-transmembrane receptor like G protein-coupled receptors (GPCRs), SMO-mediated activation of the Gli family of transcription factors is not known to involve G proteins. However, data from Drosophila and mammalian cell lines indicate that SMO functions as a GPCR that couples to heterotrimeric G proteins of the pertussis toxin (PTX)-sensitive Gαi class. Using genetically modified mice, we demonstrated that SMO signaling through G proteins occurred in the mammary gland in vivo. SMO-induced stimulation of proliferation was PTX-sensitive and required Gαi2, but not Gαi1, Gαi3, or activation of Gli1 or Gli2. Our findings show that activated SMO functions as a GPCR to stimulate proliferation in vivo, a finding that may have clinical importance because most SMO-targeted agents have been selected based largely on their ability to block Gli-mediated transcription.

Platelet Gi protein Gαi2 is an essential mediator of thrombo-inflammatory organ damage in mice

Platelets are crucial for hemostasis and thrombosis and exacerbate tissue injury following ischemia and reperfusion. Important regulators of platelet function are G proteins controlled by seven transmembrane receptors. The Gi protein Gα(i2) mediates platelet activation in vitro, but its in vivo role in hemostasis, arterial thrombosis, and postischemic infarct progression remains to be determined. Here we show that mice lacking Gα(i2) exhibit prolonged tail-bleeding times and markedly impaired thrombus formation and stability in different models of arterial thrombosis. We thus generated mice selectively lacking Gα(i2) in megakaryocytes and platelets (Gna(i2)(fl/fl)/PF4-Cre mice) and found bleeding defects comparable to those in global Gα(i2)-deficient mice. To examine the impact of platelet Gα(i2) in postischemic thrombo-inflammatory infarct progression, Gna(i2)(fl/fl)/PF4-Cre mice were subjected to experimental models of cerebral and myocardial ischemia/reperfusion injury. In the model of transient middle cerebral artery occlusion stroke Gna(i2)(fl/fl)/PF4-Cre mice developed significantly smaller brain infarcts and fewer neurological deficits than littermate controls. Following myocardial ischemia, Gna(i2)(fl/fl)/PF4-Cre mice showed dramatically reduced reperfusion injury which correlated with diminished formation of the ADP-dependent platelet neutrophil complex. In conclusion, our data provide definitive evidence that platelet Gα(i2) not only controls hemostatic and thrombotic responses but also is critical for the development of ischemia/reperfusion injury in vivo.

Identification of the GTPase-activating protein DEP domain containing 1B (DEPDC1B) as a transcriptional target of Pitx2

Pitx2 is a bicoid-related homeobox transcription factor implicated in regulating left-right patterning and organogenesis. However, only a limited number of Pitx2 downstream target genes have been identified and characterized. Here we demonstrate that Pitx2 is a transcriptional repressor of DEP domain containing 1B (DEPDC1B). The first intron of the human and mouse DEP domain containing 1B genes contains multiple consensus DNA-binding sites for Pitx2. Chromatin immunoprecipitation assays revealed that Pitx2, along with histone deacetylase 1, was recruited to the first intron of Depdc1b. In contrast, RNAi-mediated depletion of Pitx2 not only enhanced the acetylation of histone H4 in the first intron of Depdc1b, but also increased the protein level of Depdc1b. Luciferase reporter assays also showed that Pitx2 could repress the transcriptional activity mediated by the first intron of human DEPDC1B. The GAP domain of DEPDC1B interacted with nucleotide-bound forms of RAC1 in vitro. In addition, exogenous expression of DEPDC1B suppressed RAC1 activation and interfered with actin polymerization induced by the guanine nucleotide exchange factor TRIO. Moreover, DEPDC1B interacted with various signaling molecules such as U2af2, Erh, and Salm. We propose that Pitx2-mediated repression of Depdc1b expression contributes to the regulation of multiple molecular pathways, such as Rho GTPase signaling.

Insulin secretion stimulated by L-arginine and its metabolite L-ornithine depends on Gα(i2)

Bordetella pertussis toxin (PTx), also known as islet-activating protein, induces insulin secretion by ADP-ribosylation of inhibitory G proteins. PTx-induced insulin secretion may result either from inactivation of Gα(o) proteins or from combined inactivation of Gα(o), Gα(i1), Gα(i2), and Gα(i3) isoforms. However, the specific role of Gα(i2) in pancreatic β-cells still remains unknown. In global (Gα(i2)(-/-)) and β-cell-specific (Gα(i2)(βcko)) gene-targeted Gα(i2) mouse models, we studied glucose homeostasis and islet functions. Insulin secretion experiments and intracellular Ca²⁺ measurements were used to characterize Gα(i2) function in vitro. Gα(i2)(-/-) and Gα(i2)(βcko) mice showed an unexpected metabolic phenotype, i.e., significantly lower plasma insulin levels upon intraperitoneal glucose challenge in Gα(i2)(-/-) and Gα(i2)(βcko) mice, whereas plasma glucose concentrations were unchanged in Gα(i2)(-/-) but significantly increased in Gα(i2)(βcko) mice. These findings indicate a novel albeit unexpected role for Gα(i2) in the expression, turnover, and/or release of insulin from islets. Detection of insulin secretion in isolated islets did not show differences in response to high (16 mM) glucose concentrations between control and β-cell-specific Gα(i2)-deficient mice. In contrast, the two- to threefold increase in insulin secretion evoked by L-arginine or L-ornithine (in the presence of 16 mM glucose) was significantly reduced in islets lacking Gα(i2). In accord with a reduced level of insulin secretion, intracellular calcium concentrations induced by the agonistic amino acid L-arginine did not reach control levels in β-cells. The presented analysis of gene-targeted mice provides novel insights in the role of β-cell Gα(i2) showing that amino acid-induced insulin-release depends on Gα(i2).

From dinosaurs to birds: a tail of evolution

A particularly critical event in avian evolution was the transition from long- to short-tailed birds. Primitive bird tails underwent significant alteration, most notably reduction of the number of caudal vertebrae and fusion of the distal caudal vertebrae into an ossified pygostyle. These changes, among others, occurred over a very short evolutionary interval, which brings into focus the underlying mechanisms behind those changes. Despite the wealth of studies delving into avian evolution, virtually nothing is understood about the genetic and developmental events responsible for the emergence of short, fused tails. In this review, we summarize the current understanding of the signaling pathways and morphological events that contribute to tail extension and termination and examine how mutations affecting the genes that control these pathways might influence the evolution of the avian tail. To generate a list of candidate genes that may have been modulated in the transition to short-tailed birds, we analyzed a comprehensive set of mouse mutants. Interestingly, a prevalent pleiotropic effect of mutations that cause fused caudal vertebral bodies (as in the pygostyles of birds) is tail truncation. We identified 23 mutations in this class, and these were primarily restricted to genes involved in axial extension. At least half of the mutations that cause short, fused tails lie in the Notch/Wnt pathway of somite boundary formation or differentiation, leading to changes in somite number or size. Several of the mutations also cause additional bone fusions in the trunk skeleton, reminiscent of those observed in primitive and modern birds. All of our findings were correlated to the fossil record. An open question is whether the relatively sudden appearance of short-tailed birds in the fossil record could be accounted for, at least in part, by the pleiotropic effects generated by a relatively small number of mutational events.

Novel variants in GNAI3 associated with auriculocondylar syndrome strengthen a common dominant negative effect

Auriculocondylar syndrome is a rare craniofacial disorder comprising core features of micrognathia, condyle dysplasia and question mark ear. Causative variants have been identified in PLCB4, GNAI3 and EDN1, which are predicted to function within the EDN1-EDNRA pathway during early pharyngeal arch patterning. To date, two GNAI3 variants in three families have been reported. Here we report three novel GNAI3 variants, one segregating with affected members in a family previously linked to 1p21.1-q23.3 and two de novo variants in simplex cases. Two variants occur in known functional motifs, the G1 and G4 boxes, and the third variant is one amino acid outside of the G1 box. Structural modeling shows that all five altered GNAI3 residues identified to date cluster in a region involved in GDP/GTP binding. We hypothesize that all GNAI3 variants lead to dominant negative effects.

Gαi2- and Gαi3-deficient mice display opposite severity of myocardial ischemia reperfusion injury

G-protein-coupled receptors (GPCRs) are the most abundant receptors in the heart and therefore are common targets for cardiovascular therapeutics. The activated GPCRs transduce their signals via heterotrimeric G-proteins. The four major families of G-proteins identified so far are specified through their α-subunit: Gα_i, Gα_s, Gα_q and G_12/13. Gα_i-proteins have been reported to protect hearts from ischemia reperfusion injury. However, determining the individual impact of Gα_i2 or Gα_i3 on myocardial ischemia injury has not been clarified yet. Here, we first investigated expression of Gα_i2 and Gα_i3 on transcriptional level by quantitative PCR and on protein level by immunoblot analysis as well as by immunofluorescence in cardiac tissues of wild-type, Gα_i2-, and Gα_i3-deficient mice. Gα_i2 was expressed at higher levels than Gα_i3 in murine hearts, and irrespective of the isoform being knocked out we observed an up regulation of the remaining Gα_i-protein. Myocardial ischemia promptly regulated cardiac mRNA and with a slight delay protein levels of both Gα_i2 and Gα_i3, indicating important roles for both Gα_i isoforms. Furthermore, ischemia reperfusion injury in Gα_i2- and Gα_i3-deficient mice exhibited opposite outcomes. Whereas the absence of Gα_i2 significantly increased the infarct size in the heart, the absence of Gα_i3 or the concomitant upregulation of Gα_i2 dramatically reduced cardiac infarction. In conclusion, we demonstrate for the first time that the genetic ablation of Gα_i proteins has protective or deleterious effects on cardiac ischemia reperfusion injury depending on the isoform being absent.

Disruption of G-protein γ5 subtype causes embryonic lethality in mice

Heterotrimeric G-proteins modulate many processes essential for embryonic development including cellular proliferation, migration, differentiation, and survival. Although most research has focused on identifying the roles of the various αsubtypes, there is growing recognition that similarly divergent βγ dimers also regulate these processes. In this paper, we show that targeted disruption of the mouse Gng5 gene encoding the γ₅ subtype produces embryonic lethality associated with severe head and heart defects. Collectively, these results add to a growing body of data that identify critical roles for the γ subunits in directing the assembly of functionally distinct G-αβγ trimers that are responsible for regulating diverse biological processes. Specifically, the finding that loss of the G-γ₅ subtype is associated with a reduced number of cardiac precursor cells not only provides a causal basis for the mouse phenotype but also raises the possibility that G-βγ₅ dependent signaling contributes to the pathogenesis of human congenital heart problems.

Gαi2 signaling is required for skeletal muscle growth, regeneration, and satellite cell proliferation and differentiation

We have previously shown that activation of Gαi2, an α subunit of the heterotrimeric G protein complex, induces skeletal muscle hypertrophy and myoblast differentiation. To determine whether Gαi2 is required for skeletal muscle growth or regeneration, Gαi2-null mice were analyzed. Gαi2 knockout mice display decreased lean body mass, reduced muscle size, and impaired skeletal muscle regeneration after cardiotoxin-induced injury. Short hairpin RNA (shRNA)-mediated knockdown of Gαi2 in satellite cells (SCs) leads to defective satellite cell proliferation, fusion, and differentiation ex vivo. The impaired differentiation is consistent with the observation that the myogenic regulatory factors MyoD and Myf5 are downregulated upon knockdown of Gαi2. Interestingly, the expression of microRNA 1 (miR-1), miR-27b, and miR-206, three microRNAs that have been shown to regulate SC proliferation and differentiation, is increased by a constitutively active mutant of Gαi2 [Gαi2(Q205L)] and counterregulated by Gαi2 knockdown. As for the mechanism, this study demonstrates that Gαi2(Q205L) regulates satellite cell differentiation into myotubes in a protein kinase C (PKC)- and histone deacetylase (HDAC)-dependent manner.

Understanding the basis of auriculocondylar syndrome: Insights from human, mouse and zebrafish genetic studies

Among human birth defect syndromes, malformations affecting the face are perhaps the most striking due to cultural and psychological expectations of facial shape. One such syndrome is auriculocondylar syndrome (ACS), in which patients present with defects in ear and mandible development. Affected structures arise from cranial neural crest cells, a population of cells in the embryo that reside in the pharyngeal arches and give rise to most of the bone, cartilage and connective tissue of the face. Recent studies have found that most cases of ACS arise from defects in signaling molecules associated with the endothelin signaling pathway. Disruption of this signaling pathway in both mouse and zebrafish results in loss of identity of neural crest cells of the mandibular portion of the first pharyngeal arch and the subsequent repatterning of these cells, leading to homeosis of lower jaw structures into more maxillary-like structures. These findings illustrate the importance of endothelin signaling in normal human craniofacial development and illustrate how clinical and basic science approaches can coalesce to improve our understanding of the genetic basis of human birth defect syndromes. Further, understanding the genetic basis for ACS that lies outside of known endothelin signaling components may help elucidate unknown aspects critical to the establishment of neural crest cell patterning during facial morphogenesis.

The loss of Gnai2 and Gnai3 in B cells eliminates B lymphocyte compartments and leads to a hyper-IgM like syndrome

B lymphocytes are compartmentalized within lymphoid organs. The organization of these compartments depends upon signaling initiated by G-protein linked chemoattractant receptors. To address the importance of the G-proteins Gα_i2 and Gα_i3 in chemoattractant signaling we created mice lacking both proteins in their B lymphocytes. While bone marrow B cell development and egress is grossly intact; mucosal sites, splenic marginal zones, and lymph nodes essentially lack B cells. There is a partial block in splenic follicular B cell development and a 50-60% reduction in splenic B cells, yet normal numbers of splenic T cells. The absence of Gα_i2 and Gα_i3 in B cells profoundly disturbs the architecture of lymphoid organs with loss of B cell compartments in the spleen, thymus, lymph nodes, and gastrointestinal tract. This results in a severe disruption of B cell function and a hyper-IgM like syndrome. Beyond the pro-B cell stage, B cells are refractory to chemokine stimulation, and splenic B cells are poorly responsive to antigen receptor engagement. Gα_i2 and Gα_i3 are therefore critical for B cell chemoattractant receptor signaling and for normal B cell function. These mice provide a worst case scenario of the consequences of losing chemoattractant receptor signaling in B cells.